1.Field of the Invention
The present invention relates to an apparatus and method for estimating a stability factor of combustion state in each cylinder of an internal combustion engine which is mounted on an automotive vehicle, more particularly, relates to the apparatus and method for eliminating noise components not related to or not correlated to the stability factor from among engine revolution variation components so as to improve an estimating accuracy of the combustion stability factor.
2. Description of the Background Art
A high (preferable) or low (worse) stability factor of combustions in respective cylinders of an internal combustion engine depends upon a frequency characteristic of a revolution variation of the engine (refer to FIG. 13). Therefore, the engine revolution variation is detected, from the detected variation in engine revolutions, the stability factor is estimated, and an air-fuel mixture ratio supplied to the engine is controlled so that the engine stability factor is at an acceptable level.
A Japanese Patent Application First Publication No. Showa 58-160530 published on Sept. 24, 1983 exemplifies a previously proposed engine torque variation controlling method in which the engine revolution variations are extracted via a filter which is met with a bodily sensed vibration (oscillation) characteristic and the extracted variation is feedback controlled so as to reduce the torque variation (surge) of the engine.
However, since noises not related to or not correlated to the stability factor of combustion are generally invaded into a signal indicating the engine revolution speed detected in the above-described method, there is a possibility that an erroneous control for the surge is generated with the noises picked up. Since the frequencies of the noises generated due to a working accuracy error and an installing accuracy error in an engine revolution sensor, due to an imbalance of driven road wheels, and a distortion of a propeller shaft can be predicted, it may be possible to eliminate these noises through such a filter as a band rejection filter. However, since these noises have the frequencies varied according to the engine revolution speed, it is necessary to accurately vary the frequencies to be eliminated in accordance with the engine revolution speed and, hence, it is difficult for the previously proposed engine torque variation controlling method to achieve the accurate estimation of the stability factor of combustion.
On the other hand, a Japanese Patent Application First Publication No. Heisei 2-291459 (published on Dec. 3, 1990) exemplifies a previously proposed surging prevention apparatus of the vehicle in which a surging control is halted when a natural frequency of a vehicular drive train and a revolution primary (first-order) frequency of tire wheel(s) are coincident with each other.
However, although an influence of the first-order vibration (oscillation) on the detection of the revolution variation becomes a largest when its frequency of the tire wheel revolution is coincident with the natural frequency of the vehicular drive train, the influence thereof on the detection of the revolution variation is more or less present even when they are not coincident with each other. Consequently, it is difficult for the previously proposed surging control method disclosed in the latter Japanese Patent Application Publication to improve the estimating accuracy of the combustion stability factor.